1. Field of the Invention
The invention relates to an electric lamp with a hermetically closed sealing region incorporating a molybdenum foil and to a method for its manufacture.
2. Background of the Disclosure
For the hermetic sealing of the quartz glass bulb of an incandescent lamp or a discharge lamp, it is generally known to use a metal foil seal.
For example, FIG. 1 shows a tungsten halogen lamp. Molybdenum foils 2 are inserted in the sealed pinched base regions 3, which are formed at both ends of the lamp 10. To each of the outside edges of the molybdenum foils 2 is soldered a cap pin 4 as an outer lead which extends outward from the cap or end face 3a of the base region 3. In an envelope 1 is located a filament 5, whose two ends are connected by means of inner leads 6 to the inner edges of the molybdenum foils 2.
FIG. 2 is a larger-scale view of one of the sealed base regions 3 of the lamp 10. From the outer end or face 3a of the sealed base region 3 to the outer edge or end of the molybdenum foil 2, a microscopically small cavity G extends around the cap pin 4. This cavity is formed, in any event, as a result of the different thermal expansion coefficients between the quartz glass from which the envelope 1 is made and the material of the cap pin 4. Therefore, it is not possible to prevent the formation of such cavity G.
Air, including oxygen, passes into the aforementioned cavity G around the cap pin 4. As noted, the cavity extends from the outer end 3a of the sealed base region 3 to the outer end of the molybdenum foil 2. Oxygen speeds up the oxidation of the molybdenum foil 2. This oxidation leads to premature cracking of the molybdenum foil 2, which shortens the life of the lamp 10. Such oxidation in particular becomes a problem, if the sealed base region temperature rises above 350.degree. C.
Illustrated in FIG. 3 is a hitherto adopted solution for eliminating the aforementioned deficiency. The cap pin 4 at the outer end 3a in the sealed base region 3 is smeared with a tacky, vitreous material 15. This tacky, material 15 is formed by glass powder having a low melting point. The vitreous material 15, which is subsequently melted by heating, seals the opening of the cavity G.
U.S. Pat. No. 4,835,439 describes an arrangement of a sealed base region, in which a solution of alkali metal silicate is injected into the cavity G in order to eliminate the aforementioned deficiency.
In the previously described arrangement of the sealed base region, in which the opening of the cavity G is sealed with the low melting point vitreous material 15, it would appear to be disadvantageous that when switching on the lamp, the vitreous material melts, and then solidifies when switching the lamp off. The melting and resolidification of the vitreous material leads to the formation of numerous small cracks, which, when the lamp is switched off, allow the air to penetrate into the cavity G. Thus, the hitherto provided vitreous material has not been sufficiently effective for sealing the cavity G, and consequently, a long lamp life does not result therefrom. It is also difficult, due to the tackiness of the vitreous material 15, to automate the smearing process.
The alkali metal silicate solution described in U.S. Pat. No. 4,835,439 can be relatively easily injected into the cavity G, due to its good flow behavior. However, if the temperature rises above 350.degree. C., as stated, the oxidation prevention obtained is not adequate. Another disadvantage is that it takes a relatively long time for the filled solution to dry and harden.